Grease a 9 x 13-inch glass baking dish with butter or baking spray, then fill with the slices of challah spread in two layers. Fill in all the spaces with torn pieces of bread if necessary.

Mix together the eggs, milk, sugar, vanilla, and cinnamon and pour over the challah.

Refrigerate overnight.

The next day, bring the dish to room temperature. Preheat the oven to 375°. Meanwhile, combine the ingredients for the topping (flour, brown sugar, cinnamon, and softened butter), using a fork to make a crumbly mixture.